1. Field of the Invention
The present invention relates generally to a wireless communication system and, more particularly, to the transmission of sounding reference signals from multiple transmitter antennas of a user equipment. The sounding reference signals are intended to provide, among other objectives, an estimate of the channel medium experienced by the signal from each transmitter antenna. The present invention is also directed to supporting the transmission of sounding reference signals in multiple distinct bandwidths of a communication system.
2. Description of the Art
In order for a communication system to function properly, several types of signals are supported by the communication system. In addition to data signals, which convey information content, control signals also need to be transmitted to enable proper processing of the data signals. Such signals are transmitted from User Equipments (UEs) to their serving Base Station (BS or Node B) in the UpLink (UL) of the communication system and from the serving Node B to the UEs in the DownLink (DL) of the communication system. Examples of control signals include positive or negative acknowledgement signals (ACK or NAK, respectively) that are transmitted by a UE in response to correct or incorrect data packet reception. Control signals also include Channel Quality Indication (CQI) signals, which are sent by a UE to the Node B to provide information about DL channel conditions that UE experiences. Reference Signals (RS), also known as pilots, are typically transmitted by each UE to either enable coherent demodulation for transmitted data or control signals at the Node B or, in the UL, to be used by the receiving Node B to measure UL channel conditions that the UE experiences. An RS that is used for demodulation of data or control signals is referred to as a Demodulation (DM) RS, while an RS that is used for sounding the UL channel medium, and which is typically wideband in nature, is referred to as a Sounding RS (SRS).
A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, etc. A Node B is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, or some other terminology.
UEs transmit signals conveying data or control information through a Physical Uplink Shared CHannel (PUSCH) while, in the absence of PUSCH transmission, the UEs transmit control signals through a Physical Uplink Control CHannel (PUCCH). A UE receives signals conveying data information through a Physical Downlink Shared CHannel (PDSCH), while DL control signals are conveyed through a Physical Downlink Control CHannel (PDCCH).
The UEs are assumed to transmit data or control signals over a Transmission Time Interval (TTI) which may correspond to a sub-frame having a duration of 1 millisecond (msec), for example.
FIG. 1 illustrates a block diagram of a sub-frame structure 110 for PUSCH transmission. The sub-frame includes two slots. Each slot 120 includes seven symbols used for the transmission of data and/or control signals. Each symbol 130 further includes a Cyclic Prefix (CP) in order to mitigate interference due to channel propagation effects. Some symbols in each slot may be used for RS transmission 140 to provide channel estimation and to enable coherent demodulation of a received signal. It is also possible for the TTI to have only a single slot or to have more than one sub-frame. The transmission BandWidth (BW) is assumed to include frequency resource units, which are referred to herein as Resource Blocks (RBs). For example, each RB includes NscRB=12 sub-carriers. UEs may be allocated one or more consecutive RBs 150 for PUSCH transmission and one RB for PUCCH transmission. The above values are for illustrative purposes only.
PUSCH transmission or PDSCH reception by a UE may be initiated by the reception of a corresponding Scheduling Assignment (SA) at the UE, which was transmitted by the Node B through a Downlink Control Information (DCI) format in the PDCCH. The DCI format may inform a UE about a data packet transmission by the Node B in the PDSCH (DL SA), or about a data packet transmission to the Node B (UL SA) in the PUSCH. The Node B is assumed to separately code and transmit each DCI format conveying a SA.
FIG. 2 illustrates a processing chain at the Node B for an SA transmission. The Medium Access Control (MAC) UE IDentity (UE ID), for the UE that the SA is intended for, masks the CRC of the SA codeword. This enables the reference UE to identify that the SA is intended for it. A CRC of (non-coded) SA bits 210 is computed at block 220 and then masked using an exclusive OR (XOR) operation 230 between CRC bits and a MAC UE ID 240. Specifically, XOR(0,0)=0, XOR(0,1)=1, XOR(1,0)=1, XOR(1,1)=0. The masked CRC is then appended to the SA bits in block 250, and channel coding, such as for example convolutional coding, is performed in block 260. In block 270 rate matching to the allocated PDCCH resources is performed, and interleaving and modulation are performed in block 280, before transmission of a respective control signal 290.
The UE receiver performs the reverse operations of the Node B transmitter to determine whether it has an SA. These operations are illustrated in FIG. 3. A received control signal 310 is demodulated and the resulting bits are de-interleaved in block 320. The Node B rate matching is restored in block 330, followed by decoding in block 340. SA bits 360 are then obtained after extracting CRC bits in block 350 which are then de-masked by applying an XOR operation 370 with a UE ID 380. Finally, the UE performs a CRC test in block 390. If the CRC test is passed, the UE considers the SA as valid and determines the parameters for signal reception (DL SA) or signal transmission (UL SA). If the CRC test is not passed, the UE disregards the presumed SA.
A UL SA DCI format is described with respect to Table 1. Table 1 provides information about at least some of the Information Elements (IEs) an UL SA DCI format typically contains. Additional IEs or a different number of bits for each indicative IE in Table 1 may apply. The order with which IEs appear in an UL SA DCI format is arbitrary.
TABLE 1IEs of an UL SA DCI format for PUSCH Transmission.IENumber of BitsCommentResource Allocation11Assignment of Consecutive RBs(Total of 50 RBs)TBS (MCS)5MCS LevelsNDI1New Data Indicator(synchronous UL HARQ)TPC2Power control commandsCyclic Shift Indicator3SDMA (maximum of 8 UEs)Hopping Flag1Frequency Hopping (Yes/No)CQI Request1Include CQI report (Yes/No)CRC (UE ID)16UE ID masked in the CRCTOTAL40
The first IE provides RB allocation. The UL signal transmission method is assumed to be Single Carrier Frequency Division Multiple Access (SC-FDMA). With SC-FDMA, the signal transmission BW is contiguous. For an operating BW of NRBUL RBs, the number of possible contiguous RB allocations to a UE is 1+2+ . . . +NRBUL=NRBUL(NRBUL+1)/2 and can be signaled with |log2(NRBUL(NRBUL+1)/2)| bits, where ┌ ┐ denotes the ceiling operation which rounds a number to its next higher integer. Therefore, for NRBUL=50, the IE requires 11 bits. Regardless of the transmission method, the UL SA DCI format is assumed to contain an IE for resource allocation.
The second IE provides Modulation and Coding Scheme (MCS) or Transport Block Size (TBS). With 5 bits, a total of 32 MCS or TBS can be supported. For example, the modulation may be QPSK, QAM16, or QAM64 while the coding rate may take discrete values between, for example, 1/16 and 1. Using the resource allocation information, a UE can determine the TBS from the MCS, or the reverse. Some MCS IE values may be used in conjunction with the application of Hybrid Automatic Repeat reQuest (HARQ) as is subsequently described.
The third IE is a New Data Indicator (NDI). The NDI is set to 1 if a new transport block should be transmitted, and is set to 0 if the same transport block, as in a previous transmission, should be transmitted (synchronous UL HARQ is assumed in this example).
The fourth IE provides a Transmission Power Control (TPC) command for power adjustments to the transmitted PUSCH signal and SRS signal.
The fifth IE is a Cyclic Shift (CS) indicator enabling the use of a different CS for a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence used for the DM RS transmission in the PUSCH. As is subsequently described, the use of a different CS by a different UEs can provide orthogonal multiplexing of the respective RS.
The sixth IE indicates whether the PUSCH transmission hops in frequency.
The seventh IE indicates whether a DL CQI report should be included in the PUSCH.
In order for the Node B to properly determine the RBs and MCS for PUSCH transmission from a UE, it requires a UL CQI estimate over at least a part of the operating BW. Typically, this UL CQI estimate is obtained by the UE transmitting an SRS over the scheduling BW. The SRS is transmitted in one or more UL sub-frame symbols, replacing transmission of data or control. In addition to providing a Signal-to-Interference and Noise Ratio (SINR) estimate over its transmission BW, the SRS can also serve for UL TPC and UL synchronization.
FIG. 4 shows an SRS transmission. The SRS transmission occurs in a last sub-frame symbol of every other sub-frame 460, 465, for a respective 4.3% SRS overhead. UE1 410 and UE2 420 multiplex their PUSCH transmissions in different parts of the operating BW during a first sub-frame 401, while UE2 420 and UE3 430 do so during a second sub-frame 402, and UE4 440 and UE5 450 do so during a third sub-frame 403. In some UL sub-frame symbols, UEs transmit DM RSs to enable the Node B receiver to perform coherent demodulation of the data or control signal transmitted in the remaining sub-frame symbols. For example, UE1, UE2, UE3, UE4, and UE5 transmit DM RS 415, 425, 435, 445, and 455, respectively. UEs with SRS transmission may or may not have PUSCH transmission in the same sub-frame and, if they co-exist in the same sub-frame, SRS and PUSCH transmissions may be located at different parts of the operating BW.
The RS (DM RS or SRS) is assumed to be constructed from CAZAC sequences. An example of such sequences is given by the following Equation (1):
                                          c            k                    ⁡                      (            n            )                          =                  exp          ⁡                      [                                                            j2π                  ⁢                                                                          ⁢                  k                                L                            ⁢                              (                                  n                  +                                      n                    ⁢                                                                  n                        +                        1                                            2                                                                      )                                      ]                                              (        1        )            
where L is a length of the CAZAC sequence, n is an index of a sequence element, n={0, 1, 2 . . . , L−1}, and k is a sequence index. For CAZAC sequences of prime length L, the number of sequences is L−1. Therefore, an entire family of sequences is defined as k ranges in {1, 2 . . . , L−1}. However, the sequences for RS transmission need not be generated by strictly using the above expression. As 1 RB is assumed to include NscRB=12 sub-carriers, the sequences used for RS transmission can be generated by either truncating a longer prime length (such as length 13) CAZAC sequence or by extending a shorter prime length (such as length 11) CAZAC sequence by repeating its first element(s) at the end (cyclic extension), although the resulting sequences do not strictly fulfill the definition of a CAZAC sequence. Alternatively, CAZAC sequences can be generated through a computer search for sequences satisfying the CAZAC properties.
FIG. 5 shows a transmitter structure for the DM RS or the SRS based on a CAZAC sequence. The frequency domain version of a CAZAC sequence may be obtained by applying a Discrete Fourier Transform (DFT) to its time domain version. By choosing non-consecutive sub-carriers, a comb spectrum can be obtained for either the DM RS or the SRS. A comb spectrum is useful for orthogonally multiplexing (through frequency division) overlapping SRS transmissions with unequal BWs. Such SRSs are constructed by CAZAC sequences of different lengths, which cannot be orthogonally multiplexed using different CS.
Referring to FIG. 5, a frequency domain CAZAC sequence 510 is generated, the sub-carriers in the assigned transmission BW are mapped in block 520 through control of transmission bandwidth in block 530, the Inverse Fast Fourier Transform (IFFT) is performed in block 540, the CS is applied in block 550, the CP is applied in block 560 and filtering is applied in time windowing block 570 to a transmitted signal 580. The UE applies no padding in sub-carriers in which the DM RS or the SRS is not transmitted, such as in sub-carriers used for signal transmission by another UE and in guard sub-carriers (not shown). Additional transmitter circuitry such as a digital-to-analog converter, analog filters, amplifiers, and transmitter antennas, as they are known in the art, are not shown.
At the receiver, the inverse (complementary) transmitter functions are performed. This is conceptually illustrated in FIG. 6 where the reverse operations of those in FIG. 5 apply. In FIG. 6, an antenna receives a Radio-Frequency (RF) analog signal and after passing through further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters) a resulting digital received signal 610 passes through a time windowing unit 620 and the CP is removed in block 630. Subsequently, the CS of the transmitted CAZAC-based sequence is restored in block 640, a Fast Fourier Transform (FFT) is applied in block 650, the selection of the transmitted sub-carriers is performed in block 665 through control of reception bandwidth in block 660, and correlation with a CAZAC-based sequence replica 680 is applied at multiplier 670. Finally, output 690 is obtained and can then be passed to a channel estimation unit, such as a time-frequency interpolator, or an UL CQI estimator.
Different CSs of a CAZAC sequence provide orthogonal sequences. Therefore, different CSs of a CAZAC sequence can be allocated to different UEs and achieve orthogonal multiplexing of the RS transmitted by these UEs in the same RBs. This principle is illustrated in FIG. 7. In order for multiple CAZAC sequences 710, 730, 750, and 770, generated respectively from multiple CSs 720, 740, 760, and 780, of the same root CAZAC sequence to be orthogonal, CS value Δ 790 should exceed the channel propagation delay spread D (including a time uncertainty error and filter spillover effects). If TS is the duration of one symbol, the number of CSs is equal to the mathematical floor of the ratio TS/D.
The SRS transmission BW may depend on a UL SINR that the UE experiences. For UEs with low UL SINR, the Node B may assign a small SRS transmission BW in order to provide a relatively large ratio of transmitted SRS power per BW unit, thereby improving the quality of the UL CQI estimate obtained from the SRS. Conversely, for UEs with high UL SINR, the Node B may assign a large SRS transmission BW since good UL CQI estimation quality can be achieved from the SRS while obtaining this estimate over a large BW.
Several combinations for the SRS transmission BW may be supported as shown in Table 2, which corresponds to configurations adopted in 3GPP E-UTRA LTE. The Node B may signal a configuration c through a broadcast channel. For example, 3 bits can indicate one of the eight configurations. The Node B may then individually assign to each UE one of the possible SRS transmission BWs mSRS,bc (in RBs) by indicating the value of b for configuration c. Therefore, the Node B can multiplex SRS transmissions from UEs in the BWs mSRS,0c, mSRS,1c, mSRS,2c, and mSRS,3c (b=0, b=1, b=2, and b=3, respectively in Table 2).
TABLE 2Example of mSRS,bc RB values for UL BW ofNRBUL RBs with 80 < NRBUL ≦ 110.SRS BWconfigurationb = 0b = 1b = 2b = 3c = 09648244c = 19632164c = 28040204c = 37224124c = 46432164c = 56020Not4Applicablec = 64824124c = 74816 84
Variation in the maximum SRS BW is primarily intended to accommodate a varying PUCCH size. The PUCCH is assumed to be transmitted at the two edges of the operating BW and to not be overlapped (interfered) with the SRS. Therefore, the larger the PUCCH size (in RBs), the smaller the maximum SRS transmission BW.
FIG. 8 further illustrates the concept of multiple SRS transmission BWs for configuration c=3 from Table 2. The PUCCH is located at two edges, 802 and 804, of the operating BW and a UE is configured SRS transmission BWs with either mSRS,03=72 RBs 812, or mSRS,13=24 RBs 814, or mSRS,23=12 RBs 816, or mSRS,33=4 RBs 818. A few RBs, 806 and 808, may not be sounded, but this usually does not affect the ability of the Node B to schedule PUSCH transmissions in those RBs, since the respective UL SINR may be interpolated from nearby RBs with SRS transmission. For SRS BWs other than the maximum, the Node B also assigns to a UE a starting frequency position of the SRS transmission.
The SRS transmission parameters are assumed to be configured for each UE by the Node B through higher layer signaling, for example, through the MAC layer or the Radio Resource Control (RRC) layer, and remain valid until re-configured again through higher layer signaling. These SRS transmission parameters may include:
the SRS transmission BW
the SRS starting BW position
the SRS transmission comb (if the SRS has a comb spectrum)
the SRS CS
the SRS transmission period (for example, one SRS transmission every 5 sub-frames)
the starting sub-frame of SRS transmission (for example, the first sub-frame in a set of 1000 sub-frames)
whether SRS hopping is enabled (SRS transmission hops in the operating BW or not).
The configuration of the SRS transmission parameters for each UE should be such that UL throughput gains are maximized while the SRS overhead is minimized. For example, a short SRS transmission period may only result in increased UL overhead if the channel remains highly correlated between two successive SRS transmissions. Conversely, a long SRS transmission period may not provide the Node B with the proper UL CQI in sub-frames between two SRS transmissions for which the channel may become highly uncorrelated.
Enabling high UL data rates and high UL spectral efficiencies requires the use of multiple UE transmitter antennas and the application of Single-User Multiple-Input Multiple-Output (SU-MIMO) methods. To obtain the potential benefits from SU-MIMO, the Node B scheduler should be provided with a channel estimate from each UE transmitter antenna. Therefore, an SRS transmission from each UE transmitter antenna is required. Moreover, since the use of SU-MIMO is often associated with a relatively high UL SINR, the SRS transmission BW from each UE transmitter antenna may be large. This reduces the SRS multiplexing capacity and results in increased UL overhead. Considering that a UE may have as many as four or even eight transmitter antennas, the UL overhead required to support SRS transmissions may become too large and offset a significant part of the SU-MIMO spectral efficiency gains.
Configuring the SRS transmission parameters to remain constant over a long time period may often result in underutilization of the respective overhead. When the UE has no data to transmit, and hence is not scheduled by the Node B, relatively frequent SRS transmissions are wasteful. When the UE has a large amount of data to transmit, and hence it often needs PUSCH scheduling, frequent SRS transmissions are required. However, this is not possible with a semi-static configuration of the SRS transmission parameters through higher layer (MAC or RRC) signaling without incurring prohibitive SRS overhead. Such conditions typically occur for services associated with traffic bursts, such as, for example, file uploading or web browsing. Fast activation of SRS transmissions and fast configuration of the SRS transmission parameters enabled through dynamic physical layer control signaling are beneficial to address such traffic models while maintaining low SRS overhead.
A dynamically configured function is one enabled through physical layer control signaling, such as for example through a DCI format, while a semi-statically configured function is one enabled through higher layer (MAC or RRC) signaling. Physical layer signaling allows for fast UE response in the order of a sub-frame period. Higher layer signaling results in slower UE response in the order of several sub-frame periods.
For a communication system having multiple UL Component Carriers (CCs), SRS transmission from a UE is assumed to be configured (through higher layer signaling) only in those UL CCs with a respective PUSCH transmission. In such cases, it is beneficial to also enable the Node B to perform, dynamic or semi-static, activation and configuration of SRS transmissions in new UL CCs where the UE is not configured PUSCH or SRS transmission. This allows the Node B to obtain information for the interference and channel conditions the UE will experience in the new UL CCs. Based on this information, the Node B may subsequently decide to also schedule PUSCH transmissions from the UE in the new UL CCs, replace an existing UL CC with a new UL CC (discontinue scheduling in an existing UL CC and begin scheduling in the new UL CC), or make no change to the existing configuration of UL CCs.